1. Field of the Invention
The present invention relates to local oscillator generation and in particular to an improved quadrature local oscillator generation that can advantageously minimize the occurrence of spurs.
2. Related Art
Direct conversion radio transceivers require local oscillator (LO) signals at the radio frequency (RF) channel frequency to down-convert received RF signals to baseband and up-convert baseband signals to RF. To avoid voltage-controlled oscillator (VCO) pulling problems, the LO signals should not be generated by a phase-locked loop (PLL) running at the RF channel center frequency (or one of its harmonics). To solve this problem, the VCO can be set to a predetermined fraction of the RF channel frequency, wherein the VCO output can be mixed with quadrature divided versions of the VCO output. The upper side band of the mixer output is selected as the LO at the RF channel frequency.
For example, FIG. 1 illustrates a conventional quadrature local oscillator (LO) generator 100 that includes a VCO 101, two mixers 102 and 103, and a divider 104. As shown in FIG. 1, VCO 101 is set to ⅔ of the RF channel frequency. A divider 104 divides the RF channel frequency by ½, thereby generating ⅓ RF at its outputs. Mixers 102 and 103 each receive the VCO output (⅔ RF) as well as the quadrature divided versions of the VCO output (⅓ RF). Therefore, mixers 102 and 103 can generate an RF LO I and an RF LO Q, respectively, at the RF channel frequency by choosing the upper sideband (i.e. ⅔ RF+⅓ RF). Unfortunately, this configuration produces spurs due to the unwanted lower sideband in the LO output, which must be suppressed by filtering or an image rejection mixer (not shown). Therefore, a need arises for a quadrature LO generator that minimizes the occurrence of spurs.